The present invention relates to a transmission system comprising a primary terminal comprising a transmitter coupled via a transmission channel to a secondary terminal, the secondary terminal comprises channel estimation means for determining at least one transmission property of the transmission channel, the secondary terminal further comprises signal conditioning means for conditioning a signal in said secondary terminal.
The present invention is also related to a receiver, signal conditioning means and a transmission and receiving method.
A transmission system according to the preamble is known from the book xe2x80x9cDigital Communicationxe2x80x9d, by E. A. Lee and D. G. Messerschmitt, chapter 9, xe2x80x9cAdaptive Equalizationxe2x80x9d, pp. 371-378.
In transmission systems it can happen that the transmission channel has severe impairments making transmission of information without taking additional measures impossible. In such transmission systems it is often required that the receiver comprises signal-conditioning means, which are arranged for conditioning the signal, received from the transmission medium. Such signal conditioning means can comprise an equalizer for equalizing the transfer function of the transmission medium. Such an equalizer is e.g. required when the signal received by the secondary terminal comprises echoes. These echoes can be caused by multipath transmission in a terrestrial transmission system or by incorrectly terminated coaxial cables in a CATV system.
The signal conditioning means can be realized in the time domain, e.g. by means of a transversal filter, or in the frequency domain, by performing an FFT, processing the FFT coefficients and subsequently performing an inverse FFT.
An object of the present invention is to provide a transmission system according to the preamble being improved in several aspects.
Therefor the present invention is characterized in that the secondary terminal comprising setting means for setting parameters of the signal conditioning means in dependence on a previously determined value of the transmission property.
By setting the parameters of the signal conditioning means in dependence on the previously determined transmission property, it becomes possible to prepare the signal conditioning means for different circumstances such as a different frequency of the signal to be received. It is also possible to set the signal conditioning means used for conditioning a signal to be transmitted by the secondary terminal in dependence on the transmission property determined from the presently received signal.
The present invention is based on the recognition that if some information about the characteristics of the transmission channel is known, it is possible to determine from the transmission property the setting of the signal conditioning means. This information can be exploited to set the (initial) value of the coefficients of the signal conditioning means to more adequate value than is possible without using the information about the characteristics of the transmission channel.
An embodiment of the invention is characterized in that the secondary terminal comprises a receiver with adjustable tuning means for tuning the receiver to the frequency of the signal received from the transmission medium, in that the signal conditioning means are arranged for conditioning the signal received from the transmission medium, in that the tuning means are arranged for switching to the further frequency, and in that the setting means are arranged for setting the parameters of the signal conditioning means for said further frequency at switching to said further frequency.
The advantage of this embodiment is that the signal conditioning means are prepared for switching to a new frequency, resulting in the ability to perform a frequency change rapidly. This is in particular important for digital TV receivers enabling fast xe2x80x9czappingxe2x80x9d.
An embodiment of the invention is characterized in that the signal conditioning means are arranged as adaptive signal conditioning means, and in that the channel estimation means are arranged for determining the channel property from the parameters of the signal conditioning means after the parameters have converged to a final value.
An easy way of operating the channel estimation means is to take over the coefficients of the adaptive signal conditioning means after it has converged to a final state.
A further embodiment of the invention is characterized in that the setting means are arranged for storing the parameters of the signal conditioning means for a plurality of frequencies to which the receiver is tunable, and in that the setting means are arranged for setting the parameters of the signal conditioning means to the parameters previously stored for said frequency.
By storing the coefficients of the equalizer for later use with the same frequency, an easy way of determining the initial coefficients of the equalizer is obtained. It is observed that in this case, it is advantageous to use an adaptive equalizer operating according to the sign algorithm. An equalizer using the sign algorithm converges quicker than an equalizer using the LMS algorithm when the initial error is below a threshold value. If good starting values are available such as in the present invention, the initial error will in general smaller than said threshold value.
An alternative embodiment of the invention is characterized in that the secondary terminal comprises a transmitter for transmitting a signal on a frequency differing from the frequency of the signal received from the transmission medium and in that the signal conditioning means are arranged for conditioning the signal transmitted by the transmitter.
With this embodiment it becomes possible to determine the setting of signal conditioning means used for processing of a signal to be transmitted. By determining the coefficients for the signal conditioning means from the channel property determined by the channel estimation means, correct parameters for the signal conditioning means are obtained. This embodiment is based on the condition that there is a relation between the channel properties for the frequency for which the channel properties are determined, and the channel properties for the frequency of the signal to be transmitted. This is in particular the case for a CATV system in which the main channel impairments are caused by reflections.